Stroke has been a disease of the highest mortality rate and disability rate in China. Among the 1.4 billion people in China, 16 million people die of stroke each year, and the mortality rate is 1.142%. The medical expenditure of stroke is huge, which brings heavy burdens to China. In 2004, the average hospitalization expense of Chinese stroke patients is RMB 6356, which is two times of the average annual income of urban and rural residents. Currently, the treatment expense of Chinese stroke patients is RMB 40 billion per year, which is ten times of the sum of the treatment expenses of other cardiovascular diseases. For this reason, early evaluation, early diagnosis and early prevention of stroke are particularly important.
Researches show that the main cause of stroke is that atherosclerosis vulnerable plaques rupture and the ruptured plaques form thrombose which cause a downstream vessel occlusion. In China, ischemic stroke is dominant, which occupies a proportion of about 80% of all strokes. Ischemic stroke is mainly caused by the lesions of intracranial artery (46.6%) and carotid artery (30%). Thus, the key of the early prevention and the precise treatment of stroke is to accurately recognize the structures and pathological characteristics of the intracranial and carotid artery plaques.
Magnetic Resonance (MR) vessel wall imaging is the currently unique means for noninvasively and panoramically displaying joint head and neck vessel walls and plaques. MR vessel wall imaging can noninvasively and accurately recognize the compositions and inflammatory activities in the plaques, and effectively evaluate the stability and vulnerability of the plaques, thus it is hopeful to bring new breakthrough to the early warning and diagnosis of stroke.
Dr. Chung Yiu-cho of Shenzhen Institutes of Advanced Technology cooperated with the Wasserman team of Johns Hopkins University, and proposed, for the first time, to realize the intracranial artery three-dimensional vessel wall imaging using the three-dimensional fast spin echo (SPACE) technique with T1 weighting (High Resolution 3D Intracranial Imaging at 3.0T, Proceedings of the 12th Annual Meeting of ISMRM, year 2010, page 2255), in the annual meeting of the International Society for Magnetic Resonance in Medicine (ISMRM). SPACE technique was developed from the fast spin echo (Optimized three-dimensional fast-spin-echo MRI, Journal of magnetic resonance imaging, year 2014, Issue 4, Volume 39, page 745), with high image acquisition efficiency, and could achieve black blood imaging without further preparing pulses. Dr. Chung Yiu-cho used T1 weighting to reduce the cerebrospinal fluid signals and make it easier to delinear the intracranial arterial vessel wall, meanwhile, they used the advanced 32-channel head radio frequency coil to increase the signal-to-noise ratio, thereby they successfully solved multiple key issues in the three-dimensional magnetic resonance intracranial artery imaging. SPACE technique was applied to the 3T Siemens imaging system to perform a T1 weighting three-dimensional magnetic resonance intracranial arterial wall imaging, thus 0.5 mm isotropous high-resolution three-dimensional black blood intracranial artery vessel wall image was obtained, with a scanning time of 10 minutes.
SPACE has intrinsic motion-sensitive characteristics, which could dephase the moving blood spins, and is widely used in the carotid artery vessel wall imaging. But the motion-sensitivity of SPACE is not that strong and a blood flow artifact will easily occur at the carotid bifurcation, thus other black blood module shall be added to more effectively suppress the blood flow signals. Recently, Mr. Wang J N combined SPACE technique with the Delay Alternating with Nutation for Tailored Excitation (DANTE) technique to suppress the cerebrospinal fluid signal and the blood flow signal at the same time, so as to increase contrast ratios between the vessel wall and the cerebrospinal fluid and the lumen (Joint blood and cerebrospinal fluid suppression for intracranial vessel wall MRI, Magnetic resonance in medicine, year 2015). As compared with the mere SPACE technique, the combination of SPACE and the DANTE can obtain better image of joint intracranial and extra cranial arterial wall. The DANTE is a new method for suppressing the signals of moving spins (DANTE-prepared pulse trains: a novel approach to motion-sensitized and motion-suppressed quantitative magnetic resonance imaging, Magnetic resonance in medicine, year 2012, Issue 5, Volume 68, page 1423), and its suppression effect on the carotid artery blood flow signal is obviously better than the other black blood techniques. Simulation shows that the DANTE can attenuate the spin signals for 80% with a moving velocity more than 0.2 cm/s, while the flow velocity of the cerebrospinal fluid is 0.1 to 2.0 cm/s, which just falls within the signal suppression range of the DANTE.
Although the DANTE can reduce the cerebrospinal fluid signals, its suppression on the cerebrospinal fluid signals is non-uniform. The DANTE causes the cerebrospinal fluid signals to be non-uniform in the reason that the DANTE is a motion-sensitive module, which can reduce more than 90% of the signal strength of a tissue with a moving velocity above 0.2 cm/s, while the signal strength of a tissue with a moving velocity below 0.2 cm/s is substantially unchanged. The cerebrospinal fluid is in a low motion state in the human brain, and its flow velocity varies in different regions within a range of 0.1 to 2 cm/s (Velocity and pressure gradients of cerebrospinal fluid assessed with magnetic resonance imaging, J Neurosurg, year 2014, Issue 1, Volume 120, page 218). Thus the DANTE achieves different degrees of signal suppression for the cerebrospinal fluid in different region of the brain, causing the cerebrospinal fluid signals non-uniform.
The non-uniform cerebrospinal fluid signals greatly affect the precise recognition of the intracranial vessel wall. Thus, how to uniformly and effectively suppress the cerebrospinal fluid signals becomes a key issue for further improving the vessel wall imaging quality.